Capacitive transducers are used in many settings to detect pressure. Typically, a sensor includes a port connected to a region whose pressure is being measured. The sensor port is coupled to a chamber within the sensor. An electrically conductive flexible diaphragm is typically disposed within the chamber such that it is deflected by pressures applied to its face. One or more conductive electrodes can be located in proximity to the flexible diaphragm to form one or more capacitances between the electrodes and the diaphragm. As the chamber pressure changes, the diaphragm deflects, causing a change in the capacitances. A processing or detection circuit is typically connected to detect the change in capacitance, which is then related to the pressure in the chamber.
In one prior system, the processing circuitry includes an array or "bridge" of four diodes, referred to as a "diode quad bridge." Such a system is described in, for example, H. R. Dean and J. Dimeff, "A Diode-Quad Bridge Circuit for Use with Capacitance Transducers," Rev. Sci. Instrum., vol. 44, no. 10, October, 1973. In that processing circuit, the sensor capacitances are connected to opposite nodes of the diode bridge. An oscillator applies an AC excitation signal across the diodes, and a resulting AC signal is applied across the capacitances. An output voltage, which is indicative of the sensor capacitances, is measured across the remaining two opposite nodes of the diode bridge. Such systems are typically operated at frequencies around 10 kHz and produce output signals at low levels. Extra amplification circuitry is typically required, and a high signal-to-noise (SNR) ratio is difficult to obtain.
In another conventional approach, a transformer is used to couple an AC excitation to the sensor capacitances. The center of the transformer secondary is tapped and the signal at the tap is monitored. When the capacitances are balanced, no AC signal is present at the center tap. However, imbalance in the capacitances causes an AC signal to appear at the center tap. A synchronous rectification and detection scheme is applied to analyze the AC signal. The amplitude of the signal is used to indicate the difference in capacitance between the capacitors which is related to the pressure being measured.
In this transformer approach, a low-frequency drive signal is used because of the slow response of the transformer. The low frequency necessitates larger components which limit the usefulness of the circuit where small size is desirable.